This invention relates to improved methods and apparatus for, the detection of corrosion in subsurface oil well casing, and more particularly relates to methods and apparatus for the detection of casing corrosion by the measurement of the current density profile within the casing.
It is well known in the oil and gas industry to set steel subsurface pipe, casing, in wells prior to the well being placed into production. Electrochemical theory postulates a tendency for steel, or other metals, in an electrolytic environment, such as subsurface formations to go into solution. Such tendency will tend to vary with the composition of the environment. Hence, it would be expected that differences in potential would exist on a long continuous electrical conductor, like oil well casing, in contact with various geological formations and it would also be expected that current would flow in the casing as a consequence of these differences in potential. Current flow of this type is referred to as galvanic current.
The passage of current in either direction between the casing and the formation is accompanied by chemical changes related in a complex way to the strength of the current and its time of passage. If the casing gaines current, hydrogen gas is deposited on the metal. If the casing loses current, iron is carried into solution. In either event, the immediate environment of the casing changes. It has been shown that a discharge of one ampere of current from the casing for a one year period carries with it twenty pounds of iron.
One method of reversing the corrosion on casing caused by galvanic current flow consist of balancing the corrosive current with an equal and opposite current in order to cancel the corrosive current. This process is termed cathodic protection. In this process a direct current from an external source is used to make the entire casing string a cathode.
Prior art methods of detecting casing corrosion and monitoring the success of cathodic protection have consisted of measuring the voltage drop along an interval of casing. A typical instrument to measure the voltage drop consists of upper and lower spaced contactors. These contactors are in the form of knife-shaped edges or spiked rotating wheels and are separated by an interval which can be up to fifty feet. The contactors are biased into integral contact with the inside of the casing and record measurement of the voltage differential between the two contact points. Successive interval measurements provide a depth related log of the length of the well casing.
Such method and apparatus as the above described suffer from several short comings. The instrument requires a metal to metal contact between the tool and casing. Mill scale, rust, and wax deposits interfere with good contact and can result in a meaningless measurement. Further, to make a differential voltage measurement over a span of casing the resistance per foot of the casing must be known. Casings from different manufacturers and of different weights and sizes have different resistances making the resistance per foot a difficult turn to establish.
These and other disadvantages are overcome with the present invention by providing method and apparatus for measuring the current density profile within the casing by detecting the flux density within the borehole caused by galvanic current flow within the casing.